Launch Retrobatch at least once, and then open up your ~/Library/Application Support/Retrobatch/Plug-Ins/ folder. Then unzip the SeparateLandscapeandPortrait.retrobatchplugin.zip archive, and put it in the Plug-Ins folder. Relaunch Retrobatch.

A new node named “Separate Landscape and Portrait” will show up under the “Plug-In” category (though it may be spelled “Plugin” in your build). Drag and drop that into the canvas and see if that does the trick for you!

There’s a good chance this will break in a future release, but I’ll try and keep it updated. For the curious, here’s what the plugin looks like: